The present invention relates to a high efficiency, low mass hydraulic actuation system for mobile robotics, and to mobile platforms in general, where the absence of AC mains requires particular attention to overall actuator system efficiency.
Significant effort has been spent attempting to adapt stationary, industrial hydraulic actuation systems to mobile needs, but these systems generally have poor efficiency, being tenable only when used with a combustion engine. The state of the art solution today is to use low efficiency hydraulic servo valves. While these valves have exceptional control performance, they have very low efficiencies and are therefore ill suited to battery powered systems. Even in applications where efficiency is not a requirement, better efficiency can lead to significant energy savings and reduced heat loading.
The state of the art in mobile robotic actuators is one of two varieties: (1) an electric motor coupled to each axis under control using a high ratio transmission such as a harmonic drive or ball screw; or (2) an electric motor driving a hydraulic pump in parallel with a hydraulic accumulator to create a constant pressure hydraulic supply rail and a hydraulic servo valve at each axis. Option (1) is the simpler solution but results in a high inertia at the axis because of the transmission, but this transmission is fundamental to the characteristics of electric motors and cannot be avoided until a conductor with a substantially lower resistance than copper can be used in electric motor design. Option (2) provides better performance, but at an efficiency (essentially because of the servo valves) that cannot be tolerated in a battery powered application. Although other actuators, such as electroactive polymers and pneumatic artificial muscles as well as other pneumatic or muscle like actuators, offer other solution paths, they have not yet reached a state where they can be used in intensive mobile applications. Major commercial endeavors and research platforms that are designed with commercial intent such as Honda's ASIMO, the Boston Dynamics BIG DOG, and iRobot's line of PACKBOTs, use either solution (1) or (2) above without exception.